The present invention relates to an object lens used for recording/reproducing data in/from an optical recording medium, and more particularly, it relates to an object lens for use in an optical pickup of finite conjugate type that can attain good focusing characteristics in accordance with any of two kinds of optical recording media different in the substrate thickness, and a method for designing the object lens.
A general DVD reproducing apparatus needs not only a reproducing function for a DVD but also a reproducing function for a compact disk (CD) and a reproducing or recording function for a write once CD (CD-R) that has recently been rapidly spread. Reproducing light used for reproducing data from a DVD is red laser of a wavelength of a 650 nm band, and reproducing light used for reproducing data from a CD or CD-R is infrared laser of a wavelength of a 780 nm band. Accordingly, a current DVD reproducing apparatus includes two semiconductor laser diodes, that is, a red semiconductor laser diode for generating a red laser beam and an infrared semiconductor laser diode for generating an infrared laser beam.
Also, in accordance with demands for downsizing of information equipment such as a personal computer, it is necessary to downsize a DVD reproducing apparatus. In order to downsize a DVD reproducing apparatus, it is indispensable to reduce the size and the thickness of an optical pickup. For reducing the size and the thickness of an optical pickup, the optical system of the optical pickup is simplified. As one of the methods for simplifying the optical system, compatibility between a DVD and a CD is attained by using a single object lens. Specifically, although there are differences, such as the thickness of the disk substrate, the wavelength of laser used for irradiation and the numerical aperture NA of an object lens used in the irradiation with the laser, between a DVD and a CD, a system for attaining compatibility between a DVD and a CD by using a single object lens has been proposed.
For example, according to the description of Japanese Laid-Open Patent Publication No. 11-96585, as shown in FIGS. 16A and 16B, first luminous flux Sd1 passing in the vicinity of the optical axis of an object lens is used for the recording/reproducing in a first focusing position and for the recording/reproducing in a second focusing position, second luminous flux Sd2 passing outside the first luminous flux Sd1 is mainly used for the recording/reproducing in the second focusing position, and third luminous flux Sd3 passing outside the second luminous flux Sd2 is mainly used for the recording/reproducing in the first focusing position.
In this case, a collimating lens is inserted between the object lens and laser used as the light source, so that parallel rays can enter the object lens. Therefore, the lens can be easily designed so that the numerical aperture NA and the focusing position of the object lens can be changed in accordance with any of different thicknesses of a disk substrate and different wavelengths of the laser.
However, in order to make parallel rays always enter the object lens displaced in accordance with the variation of the disk, it is necessary to use a collimating lens sufficiently large as compared with the displacement of the object lens. As a result, even when a single object lens is used, the optical system is infinite and hence the optical path is long because the collimating lens is large, which disadvantageously prevents the downsizing of the whole apparatus.
Also, for reducing the size and the thickness of an optical pickup, a finite conjugate type object lens that can be singly used in an optical system is preferably used. However, with respect to divergent rays, the angle of the light entering the object lens displaced in accordance with the variation of the disk is always changed, and therefore, in the case where the wavelength of the used light is also changed, such a lens is very difficult to design and has not been realized yet.
On the other hand, with respect to semiconductor laser diodes, integration of a red semiconductor laser diode and an infrared semiconductor laser diode has been started to realize. A current DVD reproducing apparatus includes two optical components respectively for a red semiconductor laser diode and an infrared semiconductor diode. Therefore, when the two laser diodes, that is, the red semiconductor laser diode and the infrared semiconductor laser diode, are integrated, an optical component can be shared between them, so that the size and the thickness of the optical pickup can be reduced.
For example, Japanese Laid-Open Patent Publication No. 11-186651 discloses a monolithic semiconductor laser diode array integrated on one substrate as shown in FIG. 17 as the integration of a red semiconductor laser diode and an infrared semiconductor laser diode.
In this case, a distance between the light emitting portion of the infrared semiconductor laser diode and the light emitting portion of the red semiconductor laser diode can be controlled in accordance with the accuracy in photolithography performed in semiconductor diffusion process. Therefore, as compared with the case where laser chips are assembled in a hybrid manner, each light emitting portion can be very accurately provided, so that the distance between the light emitting portions can be set to 270 μm or less.